Preparation of supported cobalt catalysts suitable for hydrogenation reactions or the Fischer-Tropsch synthesis of hydrocarbons has typically been by impregnation of soluble cobalt compounds into ‘pre-formed’ oxidic support materials or by precipitation of cobalt compounds from solution in the presence of support powders or extrudates, followed by a heating step in air and then, prior to use, activation of the catalyst by reduction of the resulting cobalt compounds in the catalyst precursors to elemental, or ‘zero-valent’ form typically using a hydrogen-containing gas stream. The heating in air converts at least some of the cobalt compounds to cobalt oxide, Co3O4. The subsequent reduction with hydrogen converts the Co3O4 to cobalt monoxide, CoO, and thence the catalytically active cobalt metal.
Impregnation methods typically rely on cobalt nitrate as it is relatively easy to manufacture at low cost. It is necessary, however, to reduce the residual nitrate (NO3) level in the catalysts to very low levels to prevent emissions of nitrogen-oxide (NOx) gases to the environment during subsequent processing. Whereas NOx abatement technology, e.g. NOx scrubbing, is commonly provided for calcination of nitrate-containing catalyst precursors, it is usually absent from catalyst reduction equipment. Furthermore, installing NOx abatement technology is generally not practical where catalyst precursors are to be reduced to the active form in-situ, e.g. in a hydrogenation or Fischer-Tropsch reactor. However, we have found that in order to reduce the nitrate content of the catalyst precursor to acceptably low levels in the final catalyst precursor during the heating step in air, it is necessary to heat the precursor in air to temperatures in excess of 500° C. Prolonged heating of the catalyst precursor at these high temperature has been found to reduce the resulting cobalt surface area of the subsequently reduced catalysts, possibly as a result of increased support-metal interactions leading to undesired formation of spinel or other complex oxides. For example, heating cobalt compounds supported on alumina in air can increase cobalt aluminate formation. In the subsequent catalyst activation, cobalt aluminate is more resistant to reduction with hydrogen than cobalt oxide, requiring prolonged reduction times or increased temperatures. Both of these can lead to reduced cobalt surface areas in the resulting catalysts. As cobalt surface area has been found to be proportional to catalyst activity, a method for the preparation of the catalyst precursor at lower temperatures, but which also reduces nitrate levels to low levels is desirable.